Petrographic and mineralisation Potentials of Precambrian Pegmatities and associated rock units of Olode area, Southwestern Nigeria

Abstract

The geology of the Olode area, south-western Nigeria was investigated petrographically and geochemically in order to elucidate the mineralisation potential of the rock units in the area. The area under study is generally underlain by granite gneiss, mica schist and pegmatites. Petrographical studies indicated prevalence of anhedral quartz (30 – 50%), plagioclase (14 – 20%), orthoclase (12 – 15%), muscovite (11 – 15%), tourmaline (6 – 10) and other minerals (8 – 11%) for the pegmatite. The high value of SiO2 and Al2O3 is consistent with the petrographical study. High values and wide range in Ba (34 – 737 ppm) and Zr (3.8 – 132.6 ppm) strongly support a mixture of igneous and sedimentaryThe bivariant plots of Rb vs K/Rb, Zn vs K/Rb and Th vs K/Rb indicated a partial series of fractionation, suggesting that the pegmatites are of rare element classes while granite gneiss and mica schist belong to the barren muscovite and rare element classes. This was supported by high ratios of K/Cs and K/Ba but low Th/U values indicating distinctively low rare metal mineralization. The plot of Na2O/Al2O3 vs K2O/Al2O3 revealed an igneous precursor for all the rock units. The negative Eu anomalies especially in the pegmatitic rock unit indicates fractionation and point toward a late metasomatic effect and their relatively weak negative Ce anomalies, also suggest their rare metal mineralization. The pegmatites, granite gneiss and mica schist of Olode area considered as barren as all the samples plotted below the Gordiyenkos and Beus’ line of mineralization.